Conventional electrophotographic methods of forming multicolor images, as described in, for example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 27537/1972, 58452/1984 and the like, are performed generally in the manner of repeating the steps comprising uniformly charging a photoreceptor, imagewise-exposing the photoreactor to a separate color light to form a latent image, color developing the latent image and transferring the formed color image onto a copying sheet of paper. In such the image forming method, diferent color toner images such as of yellow, magenta, cyan, black, and the like, are transferred to be superposed, one upon another each time when one color toner image is formed on an image carrier, onto a sheet of copying paper wound around a transfer drum to thereby form a multicolor image on the sheet.
According to the above image forming method, more abundant information can be obtained than in the case of copying in black and white alone, so that the method is favorable in this respect, but it requires a transfer drum, thus causing the apparatus therefor to be of great bulk. In addition, the method has a problem that when the different color toner images are transferred, by way of superposing one color toner image upon another, onto a sheet wound around the transfer drum, transfer doubling trouble tends to appear, thus causing the resulting image to be deteriorated in the resolution. Upon this, for example, Japanese Patent O.P.I. Publication No. 144452/1981 describes a technique for forming on an image carrier a multicolor toner image comprised of a plurality of different color toners in order that the transfer thereof can be completed only at a time and at the same time the apparatus therefor can be of a compact size.
Further, the above publication also discloses another technique in which, when forming on an image carrier a superposed multicolor toner image in serial developing processes, in order to avoid such a problem that the subsequent development disturbs the toner image formed on the image carrier by the preceding development, the preceding image's toner turns back to cause a mixed color, and so forth, an AC bias voltage is applied between the image carrier and the developer transport carrier in the developing region to thereby form an oscillating electric field to thus effect the development by a non-contact method.
In such the image forming method, however, because a plurality of toner images are superposed to be formed in the same region on an image carrier and then transferred at a time on a copying material unlike the manner of forming a multicolor image by repeating the development/transfer procedure, there occur the following problems with respect to the image transfer:
(1) It takes time from the formation of the initial toner image until the transfer, and in the meantime both the combinating force due to Van der Waals force tht acts between the foregoing toner image and the image carrier and the combining force due to the image force become strengthened, thereby making it hard to perform the image transer.
(2) Even if an exposure prior to the transfer or charge elimination prior to the transfer is made in order to weaken the combining force between the initial toner image and the image carrier, its effect is not exhibited particularly when toner images are superposedly formed because the effect is intercepted by the toner image layers formed over the initial toner image.
(3) Since the toner image is formed according to the non-contact developing manner, the toner is driven at a high speed onto the surface of the image carrier, thus further increasing the combining force due to the foregoing Van der Waals force and the combining force due to the image force.
Accordingly, where a multicolor image is formed in accordance with the above image forming method, transfer-off spots appear particularly in a solid density area of the image, thus causing the image to be an awkward image. Where a sheet of paper is used as the copying material, a transfer-off trouble according to the grain marks of paper occurs, thus forming an even more awkward image.
As the above-mentioned image carrier, there are photoreceptors having thereon a photosensitive layer formed by dispersing, e.g., ZnO, TiO.sub.2, CdS, etc., into a resin, photoreceptors comprising an amorphous selenium photosensitive layer, and photoreceptors having an organic photoconductor. Of these image carriers, in a photoreceptor such as, e.g., the OPC photoreceptor, having on the surface thereof a relatively soft photosensitive layer, when the image-forming process is repeated to superpose a plurality of toner images on the photoreceptor, especially the first toner image is strongly attached onto the surface of the photoreceptor, and even if the final multicolor toner image is tried to be electrostatically transferred onto a copying material by using, e.g., a corona discharger or the like, transfer marks or transfer-off marks appear, so that no satisfactory image transfer can be accomplished. Further, of the above-mentioned OPC photoreceptors, in double-layered photoreceptors comprising a conductive support having thereon a carrier generating layer containing a carrier generating material such as, e.g., a bisazo-type pigment or the like, and, superposed thereon, a carrier transport layer containing a carrier transport material such as an aromatic amino compound, hydrazone compound, pyrazoline compound, amine derivative or the like, the carrier transport layer as the surface layer is relatively soft, so that the aforementioned transfer trouble occurs conspicuously. The reason why such the problem occurs is considered probably because the first toner image out of a plurality of toner images is attached for a relatively long time onto the image carrier, and the image force, which in the meantime continues to function between the toner particles and the image carrier, brings the toner particles into more close contact with the surface of the image carrier, and consequently the image force becomes more and more strongly functions and at the same time Van der Waals force also strongly functions.
In addition, where a laser light modulated according to digitalized image information is used as a light source, the reversal developing method to attach a toner to an exposed region is preferably used, but in transferring onto a copying material a multicolor toner image that has been obtained by repeating such the developing process, there occurs the problem of causing a repellency phenomenon when an exposure is applied prior to the image transfer for improving the transfer efficiency.
Therefore, the transfer of the multicolor toner image formed by using the reversal developing process comes to be up against even more difficult problems.
Incidentally, the occurrence of the above-mentioned repellency phenomenon is considered due to the following mechanism: That is, in the case where a multicolor toner image is formed by repeating the reversal developing process, the recharging being made during the period up to the image transfer causes the multicolor toner image and the image carrier's surface therearound to become highly charged. Hereupon, if an exposure prior to the image transfer is given, the high charge on the toner image remains intact, but the charge on its peripheral portion of the image carrier is erased, so that a high potential difference is produced between the toner image and its peripheral area of the image carrier, whereby part of the toner image is repelled by the action of the strong line of electrostatic force therebetween, thus resulting in the toner image losing its resolution.
In the method for transferring at a time onto an image carrier such as an electrophotographic photoreceptor a multicolor toner image comprised of a plurality of superposed toner images as mentioned above, the image-forming apparatus to be used therefor has advantages that it can be of a compact type despite its plurality of developing devices being incorporated therein, forms no transferred image doubling trouble, and enables to obtain images having highly excellent resolution. On the other hand, however, the apparatus is poor in the transferrability of the toner image onto a copying material, and produces transfer-off spots, resulting in awkward images. And if the surface of the image carrier used is relatively soft as that of a photoreceptor which uses, e.g., anorganic photoconductive material, the transferrability becomes even worse. We, as a result of our continued investigation about the above-mentioned problems, have now found a method for weakening the combining force of a toner image onto the aforementioned image carrier, and for improving the transferrability.